The present application claims priority under 35 U.S.C. xc2xa7119 to French Application Serial No. 97 12432, the disclosure of which is hereby incorporated by reference.
(i) Field of the Invention
The invention relates to a process for controlling at least two production units whose outputs are connected in parallel in order to produce a common output flow which is intended to feed a consumer plant, the process controlling the common output flow as a function of a predetermined target value, the process including the steps consisting in:
measuring at least one variable representing the common output flow;
for each unit, determining a control instruction corresponding to one operating state of the unit among a finite set of predetermined operating states; and
controlling the units with the control instructions determined.
The invention applies, in particular to controlling several gas producers operating in on/off mode, in particular air compressors, whose outputs are connected in parallel in order to supply a gas under pressure to a consumer plant, in particular a supply network of a factory.
(ii) Description of the Related Art
In applications of this type, the gas demands of the consumer plant vary in the course of time, so that the common output flow produced by the compressors needs to be controlled. In particular, it is desirable for the pressure at the common output of the compressors to be kept as close as possible to a predetermined target value.
In known plant, several compressors are mounted in parallel. For example, for an average consumption of 1000 m3/hour, capable of rising to a peak of 1500 m3/hour, three compressors having a nominal rate of 500 m3/hour are installed in parallel. On average, only two compressors operate at the same time, the third compressor being inactive for most of the time.
In this type of plant, the compressors have three separate operating states. The first state corresponds to the compressor being stopped, the mechanical drive means being turned off. The second state corresponds to a disengaged or xe2x80x9cidlingxe2x80x9d state of the compressor, in which the mechanical drive means are operating but the air compression means are disengaged. The third state corresponds to an engaged or xe2x80x9cproductionxe2x80x9d state of the compressor, in which the mechanical drive means are operating and the compressor is delivering compressed air at its output.
The current processes for controlling the output pressure of compressor sets of this type essentially consist in measuring the pressure at the common output of the compressors and in comparing it with a set of thresholds for activation or deactivation of each of the compressors.
In particular, when the plant includes three compressors, the process employs six thresholds, associated in pairs with a compressor. Each compressor is thus associated with an activation threshold and a deactivation threshold. Since the thresholds each correspond to one compressor, it is always the same compressor which is deactivated first. Similarly, it is always the same compressor which operates preferentially.
In order to solve this problem, it has been proposed to permute the thresholds between the various compressors in order to attempt to distribute the operating time between the various compressors. This permutation of the thresholds is carried out with a relatively long time period, of the order of a week.
Since the thresholds are each associated with one compressor, it can be seen that the system formed by three compressors is an oscillating system, with the output pressure varying between the upper and lower thresholds. The pressure also changes significantly at the output, and this leads to excessive energy consumption.
The object of the invention is to propose a control process which limits the ranges over which the output pressure of the compressors changes, and which thus makes it possible to reduce the total energy consumption.
To this end, the invention relates to a process for controlling at least two production units, characterized in that at least two variables representing the common output flow are measured and the control instructions are determined by a fuzzy-logic controller implementing the steps consisting in:
on the basis of measured variables, constructing input fuzzy variables;
for at least one unit, on the basis of the said input fuzzy variables and a knowledge base compiled beforehand and not depending on the intrinsic characteristics of the production units, determining a control instruction corresponding to one of the operating states among the finite set of predetermined operating states; and
applying the determined control instruction to the or each unit.
According to alternative embodiments, the process includes one or more of the following characteristics:
one of the said measured variables is an estimate of the derivative, of a predetermined order, with respect to time of another of the said measured variables;
the production units are gas compressors and the measured variables are at least the pressure and the rate of change of pressure;
it furthermore includes the steps consisting in:
comparing at least one of the two measured variables representing the emerging flow with a set of predetermined thresholds;
determining a control instruction corresponding to one of the operating states among the finite set of predetermined operating states, for at least one unit, when the or one of the said variables subjected to the comparison crosses a predetermined threshold, irrespective of the control instructions determined by the fuzzy-logic controller; and
applying the determined control instruction to at least one production unit.
when the or one of the said variables subjected to the comparison crosses a predetermined threshold, a delay is initiated and the determined control instruction is only applied to one of the production units at the end of the delay period if the variable in question is still beyond the said predetermined threshold;
during the delay period, the rate of change of the variable which crossed the predetermined threshold is compared with a reference threshold, and the determined control instruction is applied before the end of the delay period if the said rate of change crosses the reference threshold;
for at least one operating state, the total operating time of each production unit is stored, and the production unit to which the determined control instruction, associated with the or each state, is applied is selected from the production units totalling the shortest total operating time in the operating state resulting from application of the control instruction;
for at least one operating state, the total operating time of each production unit is stored, and permutations of the operating states between the units are made when the difference between the total operating times of the units in the operating states in question exceeds a predetermined threshold;
the input fuzzy variables are established on the basis of curves defined throughout the universal set of the measured variables, these curves all being of similar shape and regularly distributed over the universal set;
the curves are of substantially triangular shape in their non-zero zone; and
the said finite set of operating states of each unit includes an idling state and a stop state, and the idling time of a unit, before a stop, is commensurately shorter as the number of stops of the said unit which have already been made during a determined preceding time period is low.
The invention will be understood more clearly on reading the following description, which is given solely by way of example and with reference to the appended drawings;